<template>
  <canvas id="bg-canvas" ref="canvas"></canvas>
</template>

<script>
let timer = null;
export default {
  mounted() {
    //定义画布宽高和生成点的个数
    var WIDTH = this.$refs.canvas.offsetWidth,
      HEIGHT = this.$refs.canvas.offsetHeight,
      POINT = 35;

    var canvas = document.getElementById("bg-canvas");
    canvas.width = WIDTH;
    canvas.height = HEIGHT;
    var context = canvas.getContext("2d");
    context.strokeStyle = "rgba(0,0,0,0.02)";
    context.strokeWidth = 1;
    context.fillStyle = "rgba(0,0,0,0.05)";
    var circleArr = [];

    //线条：开始xy坐标，结束xy坐标，线条透明度
    function Line(x, y, _x, _y, o) {
      this.beginX = x;
      this.beginY = y;
      this.closeX = _x;
      this.closeY = _y;
      this.o = o;
    }
    //点：圆心xy坐标，半径，每帧移动xy的距离
    function Circle(x, y, r, moveX, moveY) {
      this.x = x;
      this.y = y;
      this.r = r;
      this.moveX = moveX;
      this.moveY = moveY;
    }
    //生成max和min之间的随机数
    function num(max, min=0) {
      // var min = arguments[1] || 0;
      return Math.floor(Math.random() * (max - min + 1) + min);
    }
    // 绘制原点
    function drawCricle(cxt, x, y, r, moveX, moveY) {
      var circle = new Circle(x, y, r, moveX, moveY);
      cxt.beginPath();
      cxt.arc(circle.x, circle.y, circle.r, 0, 2 * Math.PI);
      cxt.closePath();
      cxt.fill();
      return circle;
    }
    //绘制线条
    function drawLine(cxt, x, y, _x, _y, o) {
      var line = new Line(x, y, _x, _y, o);
      cxt.beginPath();
      cxt.strokeStyle = "rgba(0,0,0," + o + ")";
      cxt.moveTo(line.beginX, line.beginY);
      cxt.lineTo(line.closeX, line.closeY);
      cxt.closePath();
      cxt.stroke();
    }
    //初始化生成原点
    function init() {
      circleArr = [];
      for (var i = 0; i < POINT; i++) {
        circleArr.push(
          drawCricle(
            context,
            num(WIDTH),
            num(HEIGHT),
            num(15, 2),
            num(10, -10) / 40,
            num(10, -10) / 40
          )
        );
      }
      draw();
    }

    //每帧绘制
    function draw() {
      context.clearRect(0, 0, canvas.width, canvas.height);
      for (let i = 0; i < POINT; i++) {
        drawCricle(context, circleArr[i].x, circleArr[i].y, circleArr[i].r);
      }
      for (let i = 0; i < POINT; i++) {
        for (let j = 0; j < POINT; j++) {
          if (i + j < POINT) {
            let A = Math.abs(circleArr[i + j].x - circleArr[i].x),
              B = Math.abs(circleArr[i + j].y - circleArr[i].y);
            let lineLength = Math.sqrt(A * A + B * B);
            let C = (1 / lineLength) * 7 - 0.009;
            let lineOpacity = C > 0.03 ? 0.03 : C;
            if (lineOpacity > 0) {
              drawLine(
                context,
                circleArr[i].x,
                circleArr[i].y,
                circleArr[i + j].x,
                circleArr[i + j].y,
                lineOpacity
              );
            }
          }
        }
      }
    }

    //调用执行
    init();
    timer = setInterval(function () {
      for (var i = 0; i < POINT; i++) {
        var cir = circleArr[i];
        cir.x += cir.moveX;
        cir.y += cir.moveY;
        if (cir.x > WIDTH) cir.x = 0;
        else if (cir.x < 0) cir.x = WIDTH;
        if (cir.y > HEIGHT) cir.y = 0;
        else if (cir.y < 0) cir.y = HEIGHT;
      }
      draw();
    }, 60);
  },
  beforeDestroy() {
    if (timer) {
      clearInterval(timer);
    }
  },
};
</script>

<style scoped>
#bg-canvas {
  width: 100%;
  height: 100%;
}
</style>
